Support for RSpec 3...

Here's a first implementation. I find the `rspec3?` occurences within the tests a bit ugly, yet I haven't had an idea how to run the tests cleanly with both RSpec 2 and 3. I got a tip that I could use appraisal for this. I may look into this a bit more.

The entire generating_formatters_spec.rb (e.g. rspec_integration.rb) isn't needed in RSpec 3, as writing a custom formatter became a lot easier.

What do you think?